RFR 8190235: Clarify ClassLoaderData::is_*_class_loader_data() method implementations

[harold seigel]

Hi,

Please review this small JDK-11 RFR to clarify and clean up the 
implementations of the ClassLoaderData::is_*_class_loader_data() 
implementations.  The changes primarily consist of adding some comments.

Open Webrev: 
http://cr.openjdk.java.net/~hseigel/bug_8190235/webrev/index.html

JBS Bug: https://bugs.openjdk.java.net/browse/JDK-8190235

The changes were tested with Mach 5 tier1 - tier5 tests and JTReg JDK tests.

Thanks, Harold